Engine oil is the general lubricating oil used for internal-combustion engines. In addition to lubricating the piston rings/cylinder liners, crankshaft bearings, dynamic valve mechanism, and other sliding parts in an engine, engine oil has many other functions, such as cooling the engine and cleaning, dispersing, and neutralizing the combustion products entering the crankcase. The aforementioned piston rings/cylinder liners, crankshaft bearings, dynamic valve mechanism, and other sliding parts are the places where friction and abrasion are particularly serious in the engine.
Among the aforementioned sliding parts in the engine, lubrication of the piston ring/cylinder liner and crankshaft bearing mainly belongs to the area of fluid lubrication, while lubrication of the dynamic valve mechanism, that is, the cam/tappet, mainly belongs to the area of elastic fluid lubrication—the area of mixed/boundary lubrication. The friction condition is the most serious. In particular, the shear rate in a dynamic valve system is also as high as 107-108 s−1. An engine oil should be able to withstand such severe conditions.
In particular, demand has developed in recent years for an engine oil with a significant fuel savings effect as an environmental protection measure. Although lowering the viscosity of engine oil is effective in reducing friction in the fluid lubricating area, the lubricating oil film will be destroyed on sliding surfaces in the dynamic valve system in the mixed/boundary lubricating area with a high shear rate. As a result, metal-metal contact is increased, and the increased friction can cause abrasion or seizure. Consequently, it is indispensable to form and maintain a lubricating oil film and guarantee the abrasion resistance when creating a low-viscosity engine oil.
Usually, however, an agent for increasing the viscosity index is added to engine oil in order to guarantee high abrasion resistance at high temperature and good flowability at low temperature to widen the application temperature range. High polymers are widely used as the aforementioned agent for increasing viscosity index. The high-polymer-based viscosity index improver has the typical property of such improvers, that is, a temporary viscosity decrease due to orientation, etc., occurs during operation at high speed/high load or under other high shear conditions, and irreversible viscosity decrease occurs due to molecular weight decrease as a result of chopping of the polymer molecules when the shear conditions become severe. Also, when the viscosity of an engine oil is reduced, the engine oil film itself becomes thinner, which is related to aggravation of the shear conditions. Therefore, for an engine oil in which a viscosity index improver is added, if the viscosity is reduced by simply reducing the viscosity of the base oil, it is not possible to guarantee the oil film under high shear conditions, and abrasion can easily occur.
Consequently, in order to guarantee high abrasion resistance, the amount of zinc dithiophosphate (hereinafter referred to as “ZnDTP”) or another organic metal-based phosphorous compound added as anti-abrasion agent is increased. However, since ZnDTP will poison the catalyst used to clean the exhaust gas, its amount should be reduced rather than increased.
Given this situation, a multi-grade lubricating oil with excellent fuel savings effects and heat resistance was proposed by using an alkali earth metal salt of alkyl salicylic acid and an alkali metal salt of alkyl aryl sulfonic acid in combination to create a viscosity index improver (Japanese Kokai Patent Application No. Sho 59[1984]-27994)). However, according to the application examples, the oil type used is adjusted to SAE (Society of Automotive Engineers) No. 10W-30 grade oil. Although the abrasion resistance of the dynamic valve system is improved, since the high-temperature high-shear viscosity at 150° C. (referred to as “HTHS 150° C. viscosity” hereinafter) is required to be 2.9 mPa·s or higher, the good fuel savings effect that is an object of the present invention cannot be realized.
Also, a lubricating oil composition containing 0.04-0.12 mass % of ZnDTP, measured as the phosphorous amount, and 0.8-1.8 mass % of an alkali earth metal salt of alkyl salicylic acid, measured as the amount of sulfate ash, and having an HTHS 150° C. viscosity in the range of 2.4-3.7 mPa·s was proposed as an internal-combustion engine lubricating oil composition having excellent abrasion resistance for the parts of the dynamic valve system (Japanese Kokai Patent Application No. Hei 11[1999]-315297)). However, data for only two HTHS 150° C. viscosities, 2.6 mPa·s and 3.7 mPa·s, are described in the application examples. There is no proof at all concerning whether the abrasion resistance is good enough when the HTHS 150° C. viscosity is reduced to 2.4 mPa·s.
Based on the development situation disclosed by the aforementioned patent references, it is clear that there is an urgent demand for a low-viscosity engine oil with good fuel savings effects, that is, an engine oil having an HTHS 150° C. viscosity lower than 2.6 mPa·s, and excellent abrasion resistance under conditions of high temperature and high shear rate.